Future LTE-A (Long Term Evolution-Advanced) system can support a transmission bandwidth up to 100 MHz, while in LTE standard the largest supportable transmission bandwidth is 20 MHz. Therefore, it is desired to aggregate multiple component carriers to meet the higher requirements for transmission bandwidth in LTE-A system. Carrier aggregation is a technique proposed by 3GPP in which multiple component carriers are aggregated for transmission, in order to support the higher requirements for transmission bandwidth in future mobile communication system. The introduction of carrier aggregation brings about a new opportunity and challenge in the development of communication technology.